Why does a huge, heavy ship float, but a small metal bolt sinks?

Flotation depends on average density, not just mass. A ship is mostly hollow, so its large volume encloses a lot of air, making its average density less than that of water. A solid metal bolt has a much higher average density than water, so it sinks. The ship displaces a large weight of water, generating a massive buoyant force.

Does the shape of an object affect whether it floats or sinks?

In this simplified model, shape only matters in that it determines how much volume is available to displace fluid. The decisive factor is the object's average density (mass/volume). In reality, shape can affect stability and how an object floats, but for the basic sink/float condition, average density is key.

What is 'neutral buoyancy' and how is it achieved?

Neutral buoyancy occurs when an object's average density exactly equals the fluid's density. The buoyant force perfectly balances the object's weight, causing it to remain suspended at any depth without rising or sinking. Submarines achieve this by adjusting their overall density using ballast tanks to take in or expel water.

Does the simulator show what happens to an object that is partially floating?

Yes. For a floating object, the simulator models static equilibrium: the buoyant force equals the object's weight. The volume of water displaced is just enough so that the weight of that water equals the object's weight. The object will sink into the water until ρ_fluid * V_submerged * g = m_object * g, which determines how much of it remains above the surface.

Buoyancy Simulator

Drop objects in water. Float or sink based on density.

Live graphs

About this model

Buoyancy, the upward force that allows objects to float, is governed by Archimedes' principle. This principle states that the buoyant force on an object submerged in a fluid is equal to the weight of the fluid displaced by that object. The simulator visualizes this by allowing you to drop objects of different densities into a tank of water. The key physics equation is F_b = ρ_fluid * V_displaced * g, where F_b is the buoyant force, ρ_fluid is the fluid density, V_displaced is the volume of fluid displaced, and g is gravitational acceleration. An object's fate—sinking, floating, or remaining neutrally buoyant—is determined by comparing its average density (ρ_object = mass/volume) to the density of the fluid. If ρ_object < ρ_fluid, the buoyant force exceeds the object's weight, and it floats. If ρ_object > ρ_fluid, the object sinks. If they are equal, it hovers submerged. The simulator makes several simplifications: it assumes a uniform, incompressible fluid (water) with constant density, ignores fluid viscosity and turbulence, treats objects as rigid and homogeneous, and assumes a constant gravitational field. By interacting with the model, students can directly explore the core concept that density, not just weight or size, determines flotation. They can test predictions, observe the relationship between displaced fluid volume and buoyant force, and solidify their understanding of force balance and equilibrium in fluids.

Who it's for: Middle and high school physical science or physics students being introduced to fluid mechanics, density, and forces. It is also a valuable visual tool for educators demonstrating Archimedes' principle.

Key terms

Buoyancy
Archimedes' Principle
Density
Buoyant Force
Displaced Volume
Fluid Mechanics
Force Balance
Neutral Buoyancy

How it works

A solid sphere released slightly above the water surface: buoyant force depends on the submerged volume (spherical cap near the surface). Compare ρ_object with ρ_water to see floating, sinking, or near-neutral buoyancy. The tank has a flat bottom with mild inelastic contact.

Key equations

F_b = ρ_fluid g V_submerged, W = ρ_object g V_sphere

m z̈ = F_b − W − k v (linear drag)

Frequently asked questions

Why does a huge, heavy ship float, but a small metal bolt sinks?: Flotation depends on average density, not just mass. A ship is mostly hollow, so its large volume encloses a lot of air, making its average density less than that of water. A solid metal bolt has a much higher average density than water, so it sinks. The ship displaces a large weight of water, generating a massive buoyant force.
Does the shape of an object affect whether it floats or sinks?: In this simplified model, shape only matters in that it determines how much volume is available to displace fluid. The decisive factor is the object's average density (mass/volume). In reality, shape can affect stability and how an object floats, but for the basic sink/float condition, average density is key.
What is 'neutral buoyancy' and how is it achieved?: Neutral buoyancy occurs when an object's average density exactly equals the fluid's density. The buoyant force perfectly balances the object's weight, causing it to remain suspended at any depth without rising or sinking. Submarines achieve this by adjusting their overall density using ballast tanks to take in or expel water.
Does the simulator show what happens to an object that is partially floating?: Yes. For a floating object, the simulator models static equilibrium: the buoyant force equals the object's weight. The volume of water displaced is just enough so that the weight of that water equals the object's weight. The object will sink into the water until ρ_fluid * V_submerged * g = m_object * g, which determines how much of it remains above the surface.

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About this model

How it works

Key equations

Frequently asked questions

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About this model

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Key equations

Frequently asked questions